The idea begin HNWatcher is to track keywords and users in order to: - never miss an important news about your topic/startup/product/friend/blog/... - upvote while the submission is not too old - etc...
We also store the stories, so that you can find them whenever you need (hnsearch, running the search on hn, will give you only the last 1000 items). And we also track karmas of users.
About pricing: unfortunatly, hosting + emails ( via mailgun ) are not free, so at one point we will probably have to charge for the service if we have lots of users.